The present invention relates to an information transmitting method and an information transmitting system, and, in particular to an information transmitting method and an information transmitting system for surely transmitting one piece of information to plural terminals (such a system will be hereinafter referred to as broadcast download) in a communication system that makes both multicast transmission and unicast transmission possible.
As information transmission, there are, for example, unicast transmission for transmitting a packet to separate addresses of respective terminals and multicast transmission for transmitting a packet to an address common to plural terminals. The unicast transmission has an advantage that it is possible to separately retransmit a packet to a terminal that fails to receive the packet. The unicast transmission has a disadvantage that, even when one piece of information is transmitted to plural terminals, a transmission capacity proportional to the number of receiving terminals is consumed. On the other hand, the multicast transmission has an advantage that, when one piece of information is transmitted to plural terminals, only a transmission capacity for one terminal is consumed. The multicast transmission has a disadvantage that it is impossible to separately retransmit a packet to a terminal that fails to receive the packet.
As a technique for realizing certainty of transmission, for example, a technology described in JP-A-2003-273925 is known. In the technique described in JP-A-2003-273925, for example, a master station sends a packet in multicast once and, then, waits for delivery confirmation information from slave stations. When the number of slave stations from which the delivery confirmation information is not obtained is larger than a threshold value, the master station retransmits the packet in multicast. When the number of such slave stations is smaller than the threshold value, the master station retransmits the packet in unicast. Consequently, certainty of transmission is realized.
A system for allowing a reception side to normally receive an abandoned packet without issuing a retransmission request for the abandoned packet is also known (e.g., US2004/0117498 A1).
In the broadcast download, for example, on the premise that information is surely transmitted, it is an object to minimize consumption of a transmission capacity and quickly perform transmission.
In the technique in which a master station transmits a packet in multicast and slave stations do not transmit delivery confirmation information or retransmission requests, it is impossible to guarantee that information is surely communicated to the respective slave stations. When the download broadcast is performed only by unicast, since a transmission capacity proportional to the number of slave stations is consumed, the transmission capacity is wastefully consumed compared with transmission by multicast. In the case of time division multiplex, time required for transmission increases as the number of slave stations increases.
On the other hand, when only first transmission is performed by multicast and retransmission is performed by unicast, in particular, when there are a large number of retransmission requests, at a stage of first retransmission, a problem same as the problem of the broadcast download performed only by unicast.
As in JP-A-2003-273925, when multicast and unicast is switched on the basis of delivery confirmation information from the slave stations every time transmission is performed, since the delivery confirmation information is transmitted from the slave stations every time, an up transmission capacity is suppressed. For example, since delivery confirmation information generated by the slave stations increases in proportion to the number of slave stations every time the master station transmits a packet, when there are a large number of slave station, an up transmission capacity is suppressed. Moreover, since it is necessary to provide time for waiting for delivery confirmation information, quick transmission of information is hindered. For example, since it is necessary to provide time for waiting for delivery confirmation information from the slave stations every time the master station transmits a packet, efficiency is low in terms of quickness.